This invention relates to a thermally responsive switch making and breaking an electrical circuit by engaging and disengaging a movable contact with and from a fixed contact by means of a bimetallic or trimetallic thermally responsive element which deforms when subjected to heat, and more particularly to such a thermally responsive switch disposed in an enclosed housing together with an electric motor driving a compressor of a refrigerator or air conditioning system, a refrigerant, lubricating oil and the like for protecting the motor against overheating and overcurrent in response to the temperature of the motor, refrigerant gas or the like.
Various thermally responsive switches of the type described above have been proposed. For example, Japanese Laid-open Patent Application (Kokai) No. 61-24118 discloses a thermostat comprising a fixed contact provided in the vicinity of a closed end of a cylindrical housing, a header plate welded to an open end side of the housing, a pair of terminal pins secured to the header plate for electrically insulative relation thereto, and a bimetallic thermally responsive element secured to one of the terminal pins and carrying a movable contact.
In the above-described thermostat, the fixed contact needs to be welded in the vicinity of the closed end of the deep drawn housing and accordingly, the assembling efficiency is considerably low. Moreover, it is difficult to check the positions of the contacts and assembled products for product assurance. Additionally, it is difficult to position the movable contact secured to one end of the bimetallic thermally responsive element with respect to the fixed contact disposed in the vicinity of the closed end of the header plate so that the movable contact engages and disengages from the fixed contact. Further, in the case where a resistive heating element having a resistive value larger than that of the bimetallic element is enclosed in the housing, a plurality of terminal pins need to be secured to the header plate in insulative relation thereto, which renders the thermostat large-sized.
The inventors of the present application formerly applied to the Japanese Patent Office for a utility model and the application has been assigned Published Utility Model Application (Kokoku) No. 58-40505, which discloses an enclosed thermal relay comprising first and second outer cover members composing an enclosed housing. The second housing member serves only as a lid and a fixed contact and a thermally responsive element carrying a movable contact are provided in the second housing member. Accordingly, the first housing member is rendered large-sized as compared with the thermally responsive element, which is disadvantageous from a point of view of provision of a small-sized thermally responsive switch. Further, an operative temperature of the thermally responsive element needs to be calibrated before the housing is hermetically sealed. Additionally, the housing has a configuration disadvantageous against an external force. Accordingly, the calibrated operative temperature of the thermally responsive element is likely to be changed under the influence of a shock, which entails a disadvantage in reliability of the calibrated operative temperature of the thermally responsive element. Although changes in the design of the thermally responsive switch such as increase in the thickness of the parts may be made for solution of those disadvantages, such design changes incur economical losses and the lowering of the thermal responsibility of the switch. Moreover, it is difficult to adapt the thermally responsive switch to various electric motors having different horsepowers or rated outputs.